My 11 year old daughter is setting up our 40l tank to keep guppies, this afternoon we filled it with water (conditioned) to settle and warm up, it's a sand substrate and some live plants and am planning on transfering one of my spare fliters ruunning in my cycled tank into her tank as soon as we buy the fish so that the bacteria in it don't die off, can I expect it to cycle again with this filter? (It's been in with dirty puffers who have quite a high bioload)

Should we start with just a few fish then add some more in a few weeks time,or should we put in the 8 we plan to keep in this tank straight away as the bacteria in the filter have been used to a high bioload already?

I would play it safe and only add a couple fish at first and then test the water. If you do get an instant cycle then you can go ahead and add the rest of the planned stock. For insurance you could always add tetra safe start and add all the fish at once.

Any fish tank needs time to cycle; the bacteria must appear, colonize and then multiply by binary division if there is "food" (ammonia, then nitrite) for them. The time it takes for this to occur varies from 2 to 8 weeks normally, and is dependent upon many factors.

Seeding a new tank with live bacteria helps "jump-start" the cycle, but cannot "cycle" it, per say. It merely gets the bacteria working faster. So, very few fish at first as the previous response mentioned is wise. Of course, the more seeding you do, the more fish it can handle, but to be safe I would only put in maybe 2 guppies, wait a day (minimum), then a couple more, etc.

Live plants also quicken the cycling, and the more the faster. Plants grab ammonia/ammonium very fast.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]

When I started my 29g tank I used a piece of filter media that had been in my 15g tank's filter for a month. I started with just 4 Zebra Danios and the live plants for 3 weeks, and never had any ammonia. I slowly started adding the other fish over time, and the ammonia level was always zero.